1. Field of the Invention
The present invention relates to a magnet proof magnetic fluid sealing device, and particularly to a magnet proof magnetic fluid sealing device which is used, for example, as a bearing portion of a crystal lift apparatus for preparing the single crystal of a semiconductor material.
2. Description of the Prior Art
Recently, there have been strong demands toward the increase in diameter of a silicon single crystal prepared by a crystal lift apparatus. To meet these demands, for example, there has been reported a technique of controlling the convection of molten silicon by applying a magnetic field to the molten silicon, thereby adjusting the concentration of oxygen in the molten silicon. This technique is based on the fact that oxygen regarded as one of impurities to be contained in the silicon single crystal is eluted from quartz, an essential component of a crucible, kept at high temperatures and poured into the molten silicon, and then oxygen thus eluted travels on the surface of the growing single crystal by thermal convection and eventually it is diffused and absorbed in the single crystal.
Oxygen adsorbed on the surface of the single crystal is almost evaporated; however, in the case where rapid thermal convection occurs, oxygen is not perfectly evaporated and is possibly absorbed in the single crystal.
Meanwhile, in the molten silicon applied with a magnetic field, a current is induced to thereby suppress the degree of the thermal convection; however, if the degree of thermal convection is excessively suppressed, the molten silicon is undesirably filled with oxygen. Accordingly, the degree of thermal convection must be suitably controlled.
In this regard, there has been reported a method in which a magnetic field applied to the molten silicon is desired to be in a range of about 300 to 500 mT (mili-tesla) in order to solve the above-described problem, that is, to suitably control the degree of thermal convection.
In a magnetic fluid sealing device used as a bearing portion of a rotating feed-through device of the above crystal lift apparatus for preparing the silicon single crystal, the sealing is realized using a magnetic fluid magnetized by a magnetic field which is quite independent from the external magnetic field applied for controlling the convection of molten silicon.
The magnetic fluid sealing device used under the above circumstances may cause a problem that it no longer keeps the sealing characteristic by interference from a strong external magnetic field.
Referring to FIG. 4, there will be described a prior art magnetic fluid sealing unit.
FIG. 4 is a view illustrating an essential portion of a related art magnetic fluid sealing unit.
A magnetic fluid sealing unit 10a includes, as shown in FIG. 4, a shaft 3 made of a magnetic material, a permanent magnet 1, and a pair of magnetic pole pieces 2 disposed on both sides of the permanent magnet 1 in such a manner as to hold the permanent magnet 1 therebetween. The permanent magnet 1 and the magnetic pole pieces 2 encircle the shaft 3 with micro gaps .delta. put therebetween, so as to form a magnetic circuit. And, in the magnetic circuit, the micro gaps .delta. are filled with a magnetic fluid 5.
Referring to FIG. 5 there will be described in detail another prior art magnetic fluid sealing unit.
FIG. 5 is an enlarged view illustrating an essential portion of another prior art magnetic fluid sealing unit.
The magnetic fluid 5 in the micro gaps .delta. keeps the air-tightness of a vacuum region while withstanding a difference between a pressure in the vacuum region and a pressure which is substantially equal to atmospheric pressure is applied at each end of the shaft 3. The prior art magnetic fluid sealing device having such a function is, for example, used as a bearing portion of a rotating feed-through mechanism for introducing rotation to an enclosed chamber in a vacuum environment.
The magnetic sealing function of the magnetic fluid 5 is given by a magnetic field generated by the permanent magnets 1a, 1b as shown in FIG. 5. Accordingly, when being applied with an external large magnetic field different from the above magnetic field given by the permanent magnet 1a, 1b, the magnetic fluid sealing unit is largely affected by the external magnetic field.
When the magnetic fluid sealing unit having the magnetic circuit shown in FIG. 5 is applied with an external magnetic field of about 10 mT, it may lose the magnetic fluid sealing function and often cannot keep the necessary degree of vacuum. That is to say, the sealing unit shown in FIG. 5 does not keep its sealing function even when being applied with a very weak external magnetic field.
As shown in FIG. 5, a plurality of sets of permanent magnets 1a and 1b are arranged in such a manner that the permanent magnets 1a and 1b of each set are spaced from each other with the sides thereof having the same polarity opposed to each other; and a plurality of magnetic pole pieces 2a and 2b are alternately arranged in such a manner that each of the magnetic pole pieces 2a and 2b is placed between the associated set of the permanent magnets 1a and 1b, wherein magnetic poles of the same polarity are mutually faced in said associated set. Each of cavities O.sub.1, O.sub.2, . . . is formed in the central portion of an end portion, on the shaft 3 side, of the associated one of the magnetic pole pieces 2a and 2b, and each of acute-angled peaks P.sub.1, P.sub.2, P.sub.3, P.sub.4, . . . is formed on each of both the sides of the associated one of the magnetic pole pieces 2a and 2b. In each of the magnetic pole pieces 2a and 2b, a magnetic flux is divided into two components. The divided components of the magnetic flux which have the same polarity repel one another and are spread toward both side ends of the magnetic pole piece, respectively. In this way, in the case of the magnetic pole piece 2b, for example, there is formed a closed loop of the magnetic flux passing through the peaks P3 and P4.
Accordingly, there is formed a magnetic field distribution in which magnetic fluxes are extremely acutely concentrated on the surface of the shaft 3. With such a magnetic field distribution, the magnetic fluid 5 is significantly strongly held. It is confirmed that the magnetic fluid sealing unit in which the magnetic fields for holding the magnetic fluid 5 are strong as described above is allowed to keep the above sealing function even if an external magnetic field of about 300 mT is applied to the sealing unit in the direction where the external magnetic field directly interferes with the magnetic circuit of the sealing unit. In this way, it is proved that the magnetic fluid sealing unit shown in FIG. 5 in which sets of the magnets are arranged such that each set is arranged with the sides thereof having the same polarity opposed to each other exhibits a good sealing function.
However, when an external magnetic field as strong as 300 mT or more is applied to the prior art magnetic fluid sealing unit shown in FIG. 5, the sealing unit is affected by the external magnetic field and is made difficult to keep its sealing function.
Referring to FIG. 6, there will be described a magnetic fluid sealing device for a rotating machine, which uses the magnetic fluid sealing unit shown in FIG. 5. FIG. 6 is a view illustrating the magnetic fluid sealing device for a rotating machine, which uses the magnetic fluid sealing unit shown in FIG. 5.
A magnetic fluid sealing device 50 includes a case 20 for accommodating the entire magnetic fluid sealing unit; and a shaft 3, made of a magnetic material, for transmitting rotational motion from one end 3b (atmospheric pressure side) to the other end 3a (vacuum side), which shaft is inserted into the case 20. In the cavity of the case 20 is arranged a magnetic circuit of the magnetic fluid sealing unit 10 encircling the shaft 3 inserted into the case 20; bearings 6a and 6b, arranged at both the ends of the case 20 which supports the shaft 3; an d fixtures (hereinafter, referred to as "snap rings") 7, mounted to the bearings 6a and 6b, for fixing the bearings 6a and 6b to the shaft 3 at specific positions.
After the magnetic circuit of the magnetic fluid sealing unit 10 and the like are provided in the cavity of the case 20, a screw cap 11 for fixing the magnetic circuit and the like at a specific position in the case 20 is provided at one end of the case 20 to be screwed into a screw portion 14.
In the following description of the magnetic fluid sealing device shown in FIG. 6, the one end 3a of the shaft 3 is taken as the vacuum environment and the other end 3b is taken as the atmospheric pressure environment.
When high vacuum and high cleanliness are required for the vacuum environment side, the arrangement order of bearing-magnetic circuit-bearing shown in FIG. 6 may be changed so as not to put parts on the vacuum side as much as possible.
Each of the bearings 6a and 6b for supporting the shaft 3 and the snap rings 7 for fixing the bearings 6a and 6b to the shaft 3 is made of a quenched ferromagnetic material such as a spring steel for ensuring a strength against an elastic force. In the magnetic field, these members are magnetized, possibly into magnets. Such magnetization presents a problem in terms of sealing function.
Next, there will be described an experiment in which the above magnetic fluid sealing device 50 is located in an external magnetic source composed of a large-sized electric magnet device. FIG. 7 is a view illustrating an experiment for examining the sealing function of the magnetic fluid sealing device shown in FIG. 6.
As shown in FIG. 7, the magnetic fluid sealing device 50 was arranged in a magnetic field (M.F) generated by a strong electric magnet 100 in the direction that the axis direction of the shaft 3 of the magnetic fluid sealing device 50 perpendicularly matches with the direction of the magnetic field (M.F). Since the magnetic field (M.F) is parallel to the internal magnetic field generated by the magnets in the gap portion of the sealing device 50, there occurs large mutual interference. In this experiment, when the external magnetic field of 300 mT or more was applied to the sealing device 50, the sealing effect of the magnetic fluid sealing device 50 was lost.
The loss in the sealing effect of the magnetic fluid sealing device 50 will be described in detail with reference to FIG. 8.
FIG. 8 is an exploded view of an essential portion of the magnetic fluid sealing device shown in FIG. 6.
In FIG. 8, there occurred a leakage of vacuum in the vicinity of an acute micro gap g of the snap ring 7 for fixing the bearing 6 on the vacuum side, and a leaked magnetic fluid 5a was found thereat. This is caused by a usual pressure exerted to the magnetic fluid shown by the arrow D in FIG. 8.
The above-described experiment was repeated except that the material of the case 20 was changed into a magnetic material. As a result, it was revealed that the bearing 6 was magnetized into a strong magnet, which attracted the magnetic fluid 5 in the micro gap g, so that a force was exerted in the direction opposed to the usual pressure direction D and a magnetic fluid 5b entered the magnetic pole piece 2 on the atmospheric air side.
To prevent such an inconvenience, there has been adopted a structure in which the magnetic fluid sealing device is located far from the external magnetic field in order to prevent the magnetic fluid sealing device from being affected by the external magnetic field.
Also there has been adopted a magnetic shield mechanism making use of a property of the magnetic material sufficiently allowing a magnetic flux to pass therethrough.
Referring to FIG. 10, there will be described a method of making use of such magnetic shield mechanism.
FIG. 10 is a view illustrating a magnetic fluid sealing device 50 using the magnetic shield mechanism expected to use according to conventional arts. As shown in FIG. 10, a magnetic fluid sealing device is surrounded by a magnetic material made cylindrical member 16, and the magnetic fluid sealing device 50 is subjected to vacuum exhaust in a magnetic field. The magnetic fluid sealing device 50 was measured in terms of the leakage of the perpendicular magnetic flux as shown in FIG. 7, during vacuum exhaust. As a result, it was revealed that the sealing device 50 surrounded by the magnetic material made cylindrical member 16 which serves as a magnetic shield, having a thickness of at least 3 mm more kept a sealing function even when an external magnetic field of 300 mT or more was applied to the sealing device 50; however, the sealing device 50 surrounded by the magnetic material made cylindrical member 16 having a thickness of 3 mm or less lost the sealing function when an external magnetic field of 300 mT or less was applied to the sealing device 50.
The above effective magnetic shield mechanism, however, has a problem. That is to say, the increased thickness of the magnetic material made cylindrical member 16 increases the size of the magnetic fluid sealing device 50, to thereby require a large space to mount the sealing device 50. To solve the problem, the prior art magnetic fluid sealing device has been inevitably designed to locate the magnetic fluid sealing unit as far from an external magnetic field as possible.